Statistical analysis
Qualitative data is presented with absolute and relative frequencies (%). Depending on the distribution (normal or not), quantitative data is presented with average value, standard deviation, median and intra-quatrile range. Regularity was checked using the Kolmogorov-Smirnov criterion, and graphically with the use of histograms. The correlation between data and serum vitamin D or AMPs nasal fluid levels was determined with the use of Mann-Whitney and Kruskal-Wallis criteria for qualitative factors with two or more categories respectively, as well as with the correlation coefficient Spearman’s rho for quantitative factors. Multiple linear and logarithmic regression were used, in order to investigate how the severity of bronchiolitis is affected by various potential confounders. Results are presented in β regression coefficients, Odds Ratios (ΟR) and 95% Confidence Intervals. Statistical analysis was performed with the statistical package SPSS v25 and a probability value of p<0.05 was considered statistically significant.